Ciao Joe
I made a proposal to answer your question: "How can I delete all trace of that directory and from the database?"
If the module you like doesn't install, please ask to maintainer for the reason.
By the way, from module and plugin database it appears that "online_users_map" is named "Online Users Google Map" and is a block and not a module so the package has to be dropped in moodle/blocks and not moodle/mod.
Questionnaire (plugin)
"Missing from disk"
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